1. Field of the Invention
The present application relates to an electronic ballast for operating gas discharge lamps, to which a light sensor can be connected for the purpose of monitoring the brightness in a specific physical area, and also to a special development of such a light sensor.
2. Description of the Related Art
It is known that it is possible to sample the brightness of a workplace, for example, with the aid of a light sensor, and by means of the output signal of the light sensor activate a so-called electronic ballast which, as a function of the actual brightness value delivered by the light sensor, generates a corresponding manipulated-variable value for a lamp operated by the electronic ballast. In this connection, for example, a light sensor from the firm of Philips is known that is of an elongated form and which has the internal circuit design shown in FIG. 5. The light sensor comprises a light-sensitive resistor 31 which, in accordance with FIG. 5, is connected up to a diode 32, two resistors 33 and 36, a capacitor 34 and also a transistor 35. An analog output signal is made available at the output terminals 37a and 37b of the known light sensor, in which case on account of irradiation with differing intensities of light the resistance value of the light-sensitive resistor 31 is varied so that a resistance value, which is dependent upon the light incidence, occurs at the terminals 37a and 37b.
In the case of the known light sensor, the actual brightness value measured by the light sensor can be varied in that a sleeve, which is impervious to light and is of a different length, is slipped over the elongated sensor and thus--as a function of the length of the sleeve--shades off a portion of the incident light from the light-sensitive resistor 31 to a greater or less extent. By shading off a portion of the incident light, the actual brightness value delivered by the sensor 30 is reduced so that an electronic ballast, which is connected to the light sensor, increases the brightness of a lamp which is arranged at the workplace that is being monitored by the light sensor. On the other hand, by decreasing the extent to which the incident light is shaded off from the light-sensitive resistor 31 of the light sensor 30, the actual brightness value detected by the light sensor 30 is increased so that the corresponding lamp is dimmed by means of the electronic ballast which is connected to the light sensor 30.
As a result of using sleeves of differing lengths, it is thus possible to dim the lamp illuminating the respective workplace. In the case of the known light sensor described above, it is of course necessary for sleeves of various lengths to be kept in readiness, which sleeves can, however, easily get lost.
Furthermore, in the case of the known electronic ballasts the control or regulation of the brightness of the activated lamps with the aid of the actual brightness value delivered by the light sensor, on the one hand, and also as a function of external control information, on the other hand, is problematic, since the known electronic ballasts do not know which control or regulation possibility is to be used to effect the control or regulation of the lamps, if the electronic ballast is designed in such a way that both possibilities exist.